Dark matter on trial?

The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun. According to widely accepted theories, the solar neighbourhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.

Its really hard to tell much from such a brief discussion, and barely-legible plot.... but even radially at the distance of the sun, the baryonic contribution to the halo-potential is dominant. Looking at the z-dispersion is going to be completely dominated by baryons; with huge uncertainties in even that matter distribution. Claiming this as '8-sigma' and '5-sigma' exclusions seems very unlikely.

“The amount of mass that we derive matches very well with what we see — stars, dust and gas — in the region around the Sun,” says team leader Christian Moni Bidin (Departamento de Astronomía, Universidad de Concepción, Chile). “But this leaves no room for the extra material — dark matter — that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there!”

Here is a brief summary from the research paper:

.....In conclusion, the observations point to a lack of Galactic DM at the solar position, contrary to the expectations of all the current models of Galactic mass distribution. A DM distribution very different to what it is today accepted, such as a highly prolate DM halo, is required to reconcile the results with the DM paradigm. The interpretation of these results is thus not straightforward. We believe that they require further investigation and analysis, both on the observational and the theoretical side, to solve the problems they present. We feel that any attempt to further interpret and explain our results, beyond that presented in this paper, would be highly speculative at this stage. Future surveys, such as GAIA, will likely be crucial to move beyond this point. However, as our results currently stand, we stress that, while numerous experiments seek to directly detect the elusive DM particles, our observations suggest that their density may be negligible in the solar neighborhood......

Just to help complete the bibliographyhttp://arxiv.org/abs/1009.0925The Futile Search for Galactic Disk Dark Matter
José Luis G. Pestaña, Donald H. Eckhardt
(Submitted on 5 Sep 2010)
Several approaches have been used to search for dark matter in our galactic disk, but with mixed results: maybe yes and maybe no. The prevailing approach, integrating the Poisson-Boltzmann equation for tracer stars, has led to more definitive results: yes and no. The touchstone yes analysis of Bahcall et al. (1992) has subsequently been confirmed or refuted by various other investigators. This has been our motivation for approaching the search from a different direction: applying the Virial Theorem to extant data. We conclude that the vertical density profile of the disk is not in a state of equilbrium and, therefore, that the Poisson-Boltzmann approach is inappropriate and it thereby leads to indefensible conclusions.
12 pages. Accepted for publication in ApJ Letters

Here's the abstract of the brief (4 page) paper Chronos pointed to:http://arxiv.org/abs/1204.3919
No evidence of dark matter in the solar neighborhood
C. Moni Bidin, G. Carraro, R. A. Mendez, R. Smith
(Submitted on 17 Apr 2012)
We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane,by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.
4 pages

Here's the recent longer paper by the authors:http://arxiv.org/abs/1204.3924Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood
C. Moni Bidin, G. Carraro, R. A. Mendez, R. Smith
(Submitted on 17 Apr 2012)
We estimated the dynamical surface mass density Sigma at the solar position between Z=1.5 and 4 kpc from the Galactic plane, as inferred from the kinematics of thick disk stars. The formulation is exact within the limit of validity of a few basic assumptions. The resulting trend of Sigma(Z) matches the expectations of visible mass alone, and no dark component is required to account for the observations. We extrapolate a dark matter (DM) density in the solar neighborhood of 0±1 mMsun pc-3, and all the current models of a spherical DM halo are excluded at a confidence level higher than 4sigma. A detailed analysis reveals that a small amount of DM is allowed in the volume under study by the change of some input parameter or hypothesis, but not enough to match the expectations of the models, except under an exotic combination of non-standard assumptions. Identical results are obtained when repeating the calculation with kinematical measurements available in the literature. We demonstrate that a DM halo would be detected by our method, and therefore the results have no straightforward interpretation. Only the presence of a highly prolate (flattening q>2) DM halo can be reconciled with the observations, but this is highly unlikely in LambdaCDM models. The results challenge the current understanding of the spatial distribution and nature of the Galactic DM. In particular, our results may indicate that any direct DM detection experiment is doomed to fail, if the local density of the target particles is negligible.
35 pages

All I can say right now is ... Wow, the implications of this apparently well done observational effort can be enormous, DM is a necessary hypothesis for the current cosmological model. I guess there will soon be attempts to shoot this work down, but I'm curious about the arguments that will be used.

It is possible that Pestaña Eckhardt already shot it down by showing that the method (which is an old one) is wrong. See post #5 for their abstract.

From the Moni-Bidin paper:"Sanchez-Salcedo et al. (2011) demonstrated the validity of assumption (I), that was recently questioned in the literature (Garrido Pestana & Eckhardt 2010)".
I don't think any serious astrophysicist would use an assumption that has been agreed by the community to be wrong. It would be kind of silly. Apparently Pestaña and Eckhart have only other arxiv paper that seems to put forward a slightly cranky (at first sight)origin for the origin of DM, so their dismissing a method of falsifying DM seems a bit biased.

Here is the Sanchez-Salcedo rebuttal of Pestaña Eckhardt.http://arxiv.org/abs/1103.4356On the vertical equilibrium of the local Galactic disk and the search for disk dark matter
F.J. Sanchez-Salcedo, Chris Flynn, A.M. Hidalgo-Gamez
(Submitted on 22 Mar 2011)
Estimates of the dynamical surface mass density at the solar Galactocentric distance are commonly derived assuming that the disk is in vertical equilibrium with the Galactic potential. This assumption has recently been called into question, based on the claim that the ratio between the kinetic and the gravitational energy in such solutions is a factor of 3 larger than required if Virial equilibrium is to hold. Here we show that this ratio between energies was overerestimated and that the disk solutions are likely to be in Virial equilibrium after all. We additionally demonstrate, using one-dimensional numerical simulations, that the disks are indeed in equilibrium. Hence, given the uncertainties, we find no reason to cast doubt on the steady-state solutions which are traditionally used to measure the matter density of the disk.
6 pages, 2 figures, accepted for publication in ApJ Letters

I would say that if one takes seriously these observations and the simple and widely accepted assumptions they are based on (as explained on the paper), they are as incompatible with the current models as the FTL neutrino (maybe flawed) measure. But nevertheless it is much less likely to make news headlines, it is more subtle than a photon-neutrino race and therefore not so sellable as "Einstein was wrong" journalistic stuff.

Sean Carroll has blogged about it:http://blogs.discovermagazine.com/cosmicvariance/2012/04/20/puzzles/
He's not worried, plenty of evidence for DM on various scales (he gives links), and it could be unevenly distributed. All they showed was that there was less than expected in a certain local neighborhood region. In fact when DM is mapped we see the frequently uneven distribution of clouds of it. His attitude is pretty clear. You might want to have a look.

Sean Carroll has blogged about it:http://blogs.discovermagazine.com/cosmicvariance/2012/04/20/puzzles/
He's not worried, plenty of evidence for DM on various scales (he gives links), and it could be unevenly distributed. All they showed was that there was less than expected in a certain local neighborhood region. In fact when DM is mapped we see the frequently uneven distribution of clouds of it. His attitude is pretty clear. You might want to have a look.

I thought when you talked about "expert comment" you meant someone well versed in the observational side, like a reknown astrophysicist, Carroll is actually a cosmologist/relativist and the comments he links are mainly by theoretical cosmologists that basically repeat the mantra that DM is predicted by the model at various scales rather than addressing the article directly. I see there is some questioning of he assumptions, but they don't add that these are the assumptions that astrophysical models normally use and they are backed by what is observed. No hint that they are willing to question as easily any of the assumptions of the cosmological model that are not backed by direct observation.
At this point I'm sure someone will mention the Bullet Cluster, but then they should also mention Abell 520.

I found this:
The dark matter crisis: falsification of the current standard model of cosmology
Pavel Kroupa (AIfA, Bonn)
(Submitted on 11 Apr 2012)

The current standard model of cosmology (SMoC) requires The Dual Dwarf Galaxy Theorem to be true. According to this theorem two types of dwarf galaxies must exist: primordial dark-matter (DM) dominated (type A) dwarf galaxies, and tidal-dwarf and ram-pressure-dwarf (type B) galaxies void of DM. In the model, type A dwarfs are distributed approximately spherically following the shape of the host galaxy DM halo, while type B dwarfs are typically correlated in phase-space. Type B dwarfs must exist in any cosmological theory in which galaxies interact. Only one type of dwarf galaxy is observed to exist on the baryonic Tully-Fisher plot and in the radius-mass plane. The Milky Way satellite system forms a vast phase-space-correlated structure that includes globular clusters and stellar and gaseous streams. Similar arguments apply to Andromeda. Other galaxies also have phase-space correlated satellite systems. Therefore, The Dual Galaxy Theorem is falsified by observation and dynamically relevant cold or warm DM on galactic scales cannot exist. It is shown that the SMoC is incompatible with a large set of other extragalactic observations. Other theoretical solutions to cosmological observations exist, which yield an excellent description of astronomical observations. In particular, alone the empirical mass-discrepancy-acceleration correlation constitutes convincing evidence that galactic-scale dynamics cannot be Einsteinian/Newtonian. Major problems with inflationary big bang cosmologies remain unresolved.

Regarding the last paper (Kroupa, 2012) mentioned, I've seen something about there being a large structure of globular clusters and dwarf galaxies oriented in a plane around the Milky Way which doesn't seem consistent with DM either.

Astronomers from the University of Bonn in Germany have discovered a vast structure of satellite galaxies and clusters of stars surrounding our Galaxy, stretching out across a million light years. The work challenges the existence of dark matter, part of the standard model for the evolution of the universe. PhD student and lead author Marcel Pawlowski reports the team’s findings in a paper in the journal Monthly Notices of the Royal Astronomical Society.

The Milky Way, the galaxy we live in, consists of around three hundred thousand million stars as well as large amounts of gas and dust arranged with arms in a flat disk that wind out from a central bar. The diameter of the main part of the Milky Way is about 100,000 light years, meaning that a beam of light takes 100,000 years to travel across it. A number of smaller satellite galaxies and spherical clusters of stars (so-called globular clusters) orbit at various distances from the main Galaxy.

Conventional models for the origin and evolution of the universe (cosmology) are based on the presence of ‘dark matter’, invisible material thought to make up about 23% of the content of the cosmos that has never been detected directly. In this model, the Milky Way is predicted to have far more satellite galaxies than are actually seen.

In their effort to understand exactly what surrounds our Galaxy, the scientists used a range of sources from twentieth century photographic plates to images from the robotic telescope of the Sloan Deep Sky Survey. Using all these data they assembled a picture that includes bright ‘classical’ satellite galaxies, more recently detected fainter satellites and the younger globular clusters.

“Once we had completed our analysis, a new picture of our cosmic neighbourhood emerged”, says Pawlowski. The astronomers found that all the different objects are distributed in a plane at right angles to the galactic disk. The newly-discovered structure is huge, extending from as close as 33,000 light years to as far away as one million light years from the centre of the Galaxy.

Brave words from Kroupa:"Our model appears to rule out the presence of dark matter in the universe, threatening a central pillar of current cosmological theory. We see this as the beginning of a paradigm shift, one that will ultimately lead us to a new understanding of the universe we inhabit."